Railroad traffic management systems commonly in use today are based on the so called centralized traffic control (C.T.C.) concept, adopted in the U.S.A. in 1927. The most advanced adaptation of the C.T.C. concept is the ORE/LORENTZ/BBC system now being implemented throughout Europe. This method provides for close monitoring of each vehicle progress along its itinerary from a control center and regulation of its movement by communication of control instructions through signals and other equipment installed along the track. An itinerary for each vehicle is broadly defined in advance. Electronic data processing machines are then used to coordinate the various itineraries and to define signaling and switching instructions. The method relies upon powerful monitoring and decision making apparatuses at the traffic control center, and upon extensive detection, signaling and communication hardward installed along the tracks and between each track block and the traffic control center. The bidirectional flow of information between each vehicle and the traffic control center is not only frequent, but also lengthy and complex, since the progress of each vehicle along the track is totally dependent upon the directives of the traffic control center, with practically no control left to the discretion of its conductor.
The trend towards centralized traffic control operation of railroad networks, based on extensive train location equipment along the track, has been prompted by two factors. On one hand the traditional inability of trains to accurately determine their position between stations prevented their engineers from making, on their own, the acceleration or breaking decisions necessary to reach a predetermined point on time, or to avoid collisions. On the other hand the unavailability of practical and reliable audio radio equipment severely limited communications between train crews and traffic stations. The centralized traffic control has greatly improved the efficiency of networks formerly managed by the traditional time-table method. Under time-table programming, trains were constrained to run within predetermined schedules. These schedules however, had large safety margins to guard against possible interferences with other trains. The network traffic capacity was thus severely limited. Today the time-table method of traffic regulation can still be encountered over some simple and lightly travelled circuits.
The centralized method of traffic management, however, is not without disadvantages. Besides requiring a heavy investment in equipment and maintenance work, the method has other limitations. In spite of the repetitive nature of train schedules, the progress of a particular train over a frequently travelled itinerary is seldom similar from one run to the next. The train movement is subject to random pace variations dictated by the traffic control center in function of the current traffic condition over the network. The resulting braking and acceleration maneuvers increase the fuel consumption. Furthermore, the speed variations coupled with the uncertainty of the exact location of the train within a block create security risks. These risks can only be eliminated by increasing the minimum spacing between trains, thus causing additional delay, waste of energy and lower traffic capacity.
Today's radio communications have been tremendously improved. Electronic miniaturization allows for installation of computer assisted dead-reckoning system on board each train. The task of the traffic control centers could now be safely and very efficiently alleviated by returning to the train crews (or to the train auto-piloting system) some of the track monitoring and decision making operations.